1. Field of the Invention
This invention relates, in general, to an improved apparatus for controlling the flow of current from a power source to a load, and, more particularly, to an improved semiconductor apparatus able to be fabricated in integrated form for regulating or interrupting current flow from a power source to a load and having low series on-resistance, low shunt leakage current, and improved breakdown voltage.
2. Description of the Prior Art
Semiconductor devices have long been used as series regulating elements or switches to control the flow of current from a power source to a load. Typically, the two power carrying terminals of the semiconductor device are serially connected in the "hot", or "high side" lead between the power source and load, while the control electrode of the device is provided with a control voltage or current from a driver circuit responsive to an external (control) signal. A common return, or "ground" completes the circuit.
As used herein the words "external signal" denote the external voltage or current to be applied to the control input terminal of the overall circuit or apparatus to effectuate switching or regulation of current flow between the power source and load terminal. The words "control voltage," "control current," or "gate voltage" refer to signals related to the external signal, but internal to the overall circuit or apparatus and used to turn "on" or "off" or to continuously vary the state of individual devices within the circuit or apparatus.
In its most convenient form, the regulating or switching apparatus is arranged so that the driver circuitry draws its power from the same source, that is, the power source supplies current to the driver circuitry as well as to the load. A typical application is in automotive or other transport electrical systems wherein the power source is the battery or alternator, and the semiconductor switch or regulator replaces a mechanical relay, used, for example, to feed the headlamps, landing lights, horn, brakelights, turn signals, alternator field windings, emergency flashers, or other loads.
Series regulators or switches have, in the prior art, been constructed using bipolar power transistors as the current conducting or controlling means, and other bipolar transistors as drivers. Optically coupled devices have also been used. In order to operate from the same power source, particularly where it is desired to integrate the driver and switch functions on a single semiconductor chip or substrate, it is necessary to provide a shunt path across the power supply from which can be derived the bias to switch the current conducting device into an "on", "off", or "intermediate" state in response to the external signal. This shunt, or leakage path, wastes power and, in vehicle applications, can create a significant and undesirable battery drain, even in the off-state.
Metal-oxide semiconductor (MOS) power devices can be used in place of bipolar power devices as the switch or regulator together with other MOS devices for the driver circuitry. Because of the high gate input resistance, the wasted shunt power is much reduced. The simplest arrangement for a negative common line or ground system is to use a p-channel enhancement mode metal-oxide semiconductor (PMOS) power device as the current conducting means series connected between the positive power supply terminal and the load to regulate the current therebetween. An n-channel (NMOS) enhancement mode driver device is connected from the gate of the PMOS device to the common negative (ground) line. The external signal is coupled to the gate of the NMOS driver. When the external signal biases the gate of the NMOS driver positive, above its threshold voltage, it becomes conductive thereby pulling the gate of the PMOS power device to the negative line. This turns on the PMOS power device, permitting conduction between the power source and load. The threshold voltage of a MOS device is that voltage which must be applied between the gate and source terminals to obtain significant current flow between the source-drain terminals of the device. Pull-down resistors are provided between gate and source terminals of the devices to hold the circuit in the off-state in the absence of the external signal.
This approach is simple, has a low component count, and reduces the shunt leakage. However, it requires a relatively large power device to achieve acceptably low on-resistance because of the poor carrier mobility in PMOS devices. This large device area substantially increases the overall cost of the apparatus. An NMOS power device of higher carrier mobility cannot be directly substituted because the drive circuit would not provide the correct polarity of signal to the control gate of the NMOS power device. Thus, a need continues to exist for an improved high side line switch or regulator apparatus for controlling current flow from a power source to a load, which draws power from the same source, which can be readily integrated, and which has low on-resistance and shunt leakage. High breakdown voltage is also desirable.
Accordingly, it is an object of this invention to provide an improved apparatus responsive to an external signal for regulating the flow of current from a power source to a load.
It is further an object of this invention to provide an improved high side line switching apparatus for regulating the flow of current utilizing MOS power transistors.
It is an additional object of this invention to provide a switching apparatus for regulating the flow of current which permits the use of NMOS power transistors in the positive line of a negative return system.
It is a further object of this invention to provide an MOS apparatus for regulating the flow of current having reduced series on-resistance.
It is an additional object of this invention to provide an improved apparatus for regulating the flow of current from a positive power source to a load using an NMOS power device serially connected in the positive lead and a driver circuit, fed from the same source, to control the power device.
It is a still further object of this invention to provided an improved means for holding a current regulating device in an off-state in the absence of an external signal.